Viscous material dispensers, some of which are known as caulking guns, are now well known. A dispenser described and claimed in U.S. Pat. Nos. 5,775,539 and 5,909,830 issued Jul. 7, 1998 and Jun. 8, 1999 (herein The Patents) is a high pressure dispenser that has enjoyed commercial success. The dispenser's success has been in mixing and dispensing two part materials of high viscosity. An example of such a material is that material which is used in installing replacement windshields and rear windows in automobiles.
The Patents disclose and claim a piston which when made into high precision prototypes preformed quite well when dispensing materials in tests. However, pistons of the type disclosed in The Patents when produced in quantities did not prove fully satisfactory when used with one commercially available two part adhesive having one of its to be mixed parts housed in an aluminum alloy foil “sausage” skin. When the piston of The Patents, is used with this particular two part material the sausage skin all too frequently bypasses the flexible lip of a piston and extends between the piston and a surrounding, close fitting, wall of the dispenser